When a furnace ignites briefly and shuts down, a fouled flame sensor or failing surface ignitor is usually preventing sustained burner operation. If the blower runs continuously while circulating cold air, a failed capacitor, damaged blower motor, or thermostat communication fault may be at play. Repeated short cycling typically indicates a high-limit switch tripping due to internal overheating. For heat pumps in winter, temporary cold air during a defrost cycle is normal operation, but persistent cold air suggests a failed reversing valve or sensor fault.
Gas combustion issues require immediate attention to maintain home safety. According to the U.S. Consumer Product Safety Commission, furnace heat exchanger issues like cracks are a major cause of carbon monoxide leaks in residential properties. If you notice soot near the burner assembly, loud rattling on startup, or an active CO detector, turn off the system at the breaker, ventilate the home, and call for professional service immediately.
